1. Field of the Invention
The present invention relates to a method for treating a plurality of magnetic recording media with ultraviolet rays. Each of these magnetic recording media is used in, for example, an external storage device of a computer.
2. Description of the Related Art
Lubricants for use in magnetic recording media, particularly in magnetic disks, have been developed in order to improve the durability and reliability of a magnetic recording medium by reducing a frictional force generated between a protective film of the magnetic recording medium and a magnetic head.
For example, attempts have conventionally been made to improve the lubrication characteristics of the surface layer of a magnetic disk: forming a diamond-like carbon (DLC) protective film on the surface layer and then forming on this protective film a perfluoropolyether lubricant film having a polar end group, such as a hydroxyl group, or a cyclic triphosphazene end group.
The lubricant film on the protective film consists of two layers: a layer bound to the protective layer (referred to as “bonded lubricant layer,” hereinafter), and a layer not bound to the protective layer (referred to as “free lubricant layer,” hereinafter). It is appropriate, from the perspective of the improvement of the lubrication characteristics, that the lubricant film has a thin free lubricant layer and a thick bonded lubricant layer.
However, with the recent growth in density of the magnetic disks, demands for the lubrication characteristics are becoming more and more difficult. In the future, therefore, it is inevitable to increase the maximum film thickness of the bonded lubricant layer.
Moreover, the recent hard disk drives are becoming more and more versatile, ranging from personal computers used indoors, to portable devices and car navigation systems used outdoors. Especially in an environment with high temperature and high humidity where the moisture contained in the highly humid air adheres to the disks of a hard disk drive, the moisture inhibits the magnetic head slider from floating above the disks. The problem, therefore, is how to form the lubricant film on the surface of each magnetic disk more precisely.
Increasing the film thickness of the bonded lubricant layer has been proposed as a method for solving the problem described above. In order to increase the film thickness of the bonded lubricant layer, it is said to be effective to treat each magnetic disk using ultraviolet rays during the formation of the bonded lubricant layer. In this UV treatment, instead of treating the magnetic disks one by one, the whole cassette storing the magnetic disks (e.g., a whole stack of twenty-five magnetic disks) needs to be treated, in order to improve the efficiency of the treatment and due to the steps prior to and subsequent to the UV treatment.
However, the problem in treating the whole cassette is that the film thickness of the bonded lubricant layers within the cassette varies because the waiting time of the magnetic disks for waiting to be UV-treated under a UV lamp house of an apparatus varies depending on the positions of the disks in the cassette. Whereas direct ultraviolet rays are radiated over all of the magnetic disks (e.g., a total of twenty-five magnetic disks) under the same conditions, indirect ultraviolet rays are radiated when the cassette is transported and the position of each disk is aligned with a lifter of a curtain under the UV lamp house. Therefore, a group of the magnetic disks that are treated with ultraviolet rays in the cassette around at the end (e.g., the 23rd to 25th magnetic disks out of the total of twenty-five magnetic disks) are irradiated with less indirect ultraviolet rays, reducing the bonded ratio. For this reason, the magnetic disks that are UV-treated in the cassette around at the end need to be treated in a manner that the film thickness of the bonded lubricant layers becomes equal to the film thickness of the other magnetic disks.